In drilling for petroleum or natural gas, the drilling debris, is flushed out of the borehole by means of drilling mud and is then separated from the fluid, which is recirculated. The hydraulic pressure of the drilling mud in the borehole should be somewhat greater than the reservoir pressure in order to keep the borehole open and to prevent ingress of water, oil or gas from the drilled formations. Furthermore, it is advantageous if the interaction between the drilling mud and the drilled formations is low.
Aqueous drilling muds are usually suspensions of clay in water, which may contain protective colloids and other chemicals as chemical conditioners. They offer the advantage of forming a thin but dense filter cake on drilling through porous rock. In more highly porous or fissured formations, however, this effect is no longer sufficient. Here, large amounts of drilling fluid may be lost by being forced under their own hydrostatic pressure into the interstices and fissures of the surrounding formation. To prevent these unwanted losses, "plugging agents" such as mica, cellophane chips, walnut shells and similar materials can be added to the drilling mud, with the aim of sealing the loss zone. The disadvantage of these sealing materials is, however, that they are not compression-resistant and, after they have performed their sealing task, can no longer be removed. Their removal is however essential to prevent permanent damage to the reservoir bed.
An ideal sealing material must be sufficiently fluid to penetrate into the interstices and fissures of the geological formation and, only after it has done so, must solidify to such an extent that in this state it can withstand the hydrostatic pressure of the drilling mud. Furthermore, the solidified sealing material should be easy to remove again from the interstices and fissures of the formation, if the sealed formation is a reservoir in a deposit worth extracting. The sealing material must also be suitable for precementing the borehole walls, i.e., it must effect sealing of the formation in the area of the reservoir bed before actual cementation of the string of casing. In this way, penetration of the cement into the reservoir bed and the possible resulting damage to the reservoir bed can be prevented. For actual cementation of borehole walls, "deep well cements" are used, which are made fluid and thus easy to pump by adding large amounts of mix water. The setting of such cements can be accelerated by adding calcium chloride. Usually, however, the setting of cement slurries is retarded by adding materials such as starch, cellulose or sugar waste products, and especially lignin sulphonate (cf. "Ullmanns Enzyclopadie der Technischen Chemie", 3rd Ed., Vol. 6 (1955), p. 577).
A product which sets like cement, but which is acid soluble is "Sorel's cement", a mixture of magnesium oxide, magnesium chloride and water. Trials have already been performed with Sorel's cement for cementation of borehole walls.
In these attempts, it was found that in some cases the borehole became blocked due to premature setting of the Sorel's cement. Elevated temperatures in the borehole or the heat of reaction of the setting Sorel's cement initiated and accelerated this process.
The requirement therefore arose to find a pumpable material which sets into an acid soluble solid mass both at room temperature and at elevated temperature, but only after a period of 1.5 to 4 hours.